Sequence-palaeogeography and coal accumulation of the Damoguaihe Formation in the Wujiu mining area, Yakeshi Coalfield, Hailar Basin

The Wujiu mining area of the Yakeshi Coalfield in the Hailar Basin is rich in coal resources and has a great potential for coalbed methane exploration.It has become one of the areas for coal and coalbed methane exploration in the Hailar Basin.The Lower Cretaceous Damoguaihe Formation was the main coal-bearing strata, in order to find a favorable coal accumulation zone, the authors studied its lithofacies types, sedimentary facies, sequence stratigraphy and coal accumulation characteristics based on some boreholes data.The Damoguaihe Formation was mainly composed of conglomerate, sandstone, siltstone, and mudstone, with numerous thick coal seams.Five types of sedimentary systems were identified, including alluvial fan, fan delta, braided river, braided river delta and lake.The Damoguaihe Formation could be subdivided into two third-order sequences according to the sequence boundaries represented by regional unconformity, erosional base of the fluvial channel sandstones, and inter-fluvial palaeosols.Sequence I, from the bottom of the Damoguaihe Formation to the bottom of thick sandy conglomerates, was mainly composed of conglomerate, siltstone and mudstone.The total thickness of the coal seams in this sequence could reach 13 m.The main sedimentary systems were those of the alluvial fan, fan delta and lake.Sequence II, from the bottom of thick sandy conglomerates to the top of the Damoguaihe Formation, consisted of siltstone and mudstone.The total thickness of the coal seams in this sequence was up to 20 m.The sedimentary systems were those of the fan delta, braided river, braided river delta and lake.Compared with sequence Ⅰ, sequence Ⅱ had a relatively deeper sedimentary environment, and a larger coal seam thickness.According to established sedimentary model, a single factor analysis and multi-factor comprehensive mapping method were applied to recover the palaeogeographic pattern within each third order sequence as the mapping unit.The palaeogeographic units in this area were mainly the alluvial fan and-fan delta in the east, the braided river and-braided river delta in the west, and the lacustrine in the central area.The provenance was mainly from the eastern direction of the study area.The main coal-forming environments in the study area were the littoral-sublittoral lacustrine environments.The coal seams were mainly formed in the high systems tract (HST), where the high peat accumulating rate could be well balanced by the relatively higher rate in accommodation creation.It is predicted that the thickest coal zones are distributed in the central east of the study area, and the coal seams will gradually become thinner towards the northwest and southeast.